Abstract: High molecular weight Poly (Butylene Succinate) was obtained from Succinic Acid and 1, 4-Butanediol and based on different catalysts at different conditions through a polycondensation process, the side reaction of esterification were studied. The results showed that the esterification and the side reaction were significant affected by different esterification conditions, the number average molecular weight of the synthesized PBS being from 5.7*104 g/mol to 8.0*104 g/mol.